1. Field of the Invention
The invention relates to a method and a sensor for determining the hydrocarbon dew point in a gas, particularly in a gaseous fuel.
2. Description of related art including information disclosed under 37 CFR §§1.97 and 1.98
The hydrocarbon dew point temperature is an important measured quantity for characterizing the quality of natural gas or other gaseous fuels.
A device and a method for determining the hydrocarbon dew point in a gas are known from U.S. Pat. No. 4,946,288. This specification teaches to shine light onto a bedewing surface through a measurement gas volume. Light scattered by the bedewing surface after it has passed through the measured gas again is detected by means of a CCD arrangement. The bedewing surface is cooled and it is possible to detect when the dew point temperature has been reached through a decrease in the scattered light intensity.
In the case of the device known from U.S. Pat. No. 4,946,288 the light is guided several times through the measurement gas, which requires a comparatively voluminous measurement chamber. In addition, the known device has a comparatively complicated calibration and due to the direct contact of numerous components with the measurement gas is relatively sensitive to dirtying. As a result the known device is comparatively expensive.
The company publication “Condumax—dew point analyzer for hydrocarbons” of the company Michell Instruments also discloses the so-called dark spot principle for hydrocarbon dew point temperature determination. This involves a concentrated light beam being focussed onto a surface with a conical depression and the surface temperature being varied. In the non-bedewed state a large part of the light is imaged as an annular reflection. The scattered light within said light ring is optically detected. If condensate is deposited on the surface, there is a change to the optical conditions and the intensity of the reflected light in the vicinity of the ring increases, whereas the scattered light intensity is reduced in the vicinity of the so-called dark spot. These effects can be established for the detection of bedewing. Also in the dark spot method the light is transilluminated through the measurement gas volume, which can have the above-described, undesired consequences.
DE 35 43 155 discloses an optical dew point sensor with a light conducting fibre, which has purposeful damage. However, such a light conducting fibre is comparatively damage-susceptible in operation. Moreover, it can be difficult to sufficiently accurately reproduce the desired fibre damage during manufacture, which can lead to high calibration costs.
Another dew point sensor for determining the temperature of atmospheric water is described in U.S. Pat. No. 3,528,278. In the case of this device light is guided through a crystalline prism to condensation areas placed on a planar surface of the prism and which are in contact with the measurement gas. The light reflected back into the prism from the condensation areas is measured by a light sensor. If water condensate is deposited in the condensation areas, there is a change to the critical angle for the total reflection of the light. Thus, when using bedewing there is a coupling out of light from the prism which can be detected at the light sensor as a light intensity decrease.
However, if the known dew point sensor is used for hydrocarbons, on dropping below the dew point there is frequently only a limited reflected light intensity decrease, so that measurement errors can arise or even dew point determination can be made completely impossible.
A similarly functioning dew point sensor for atmospheric water is described in German patent application DE 10 2004 038 397.9. This dew point sensor has a planar condensation surface with semihydrophobic properties. Due to the semihydrophobic surface properties condensing water droplets acquire a specific shape, which ensures a particularly effective coupling out of light on bedewing. As a result a particularly marked intensity drop in the reflected light intensity on cooling below the dew point and therefore a particularly reliable dew point determination are ensured.
However, on dropping below the dew point when such sensors are used for hydrocarbons there is also often only a limited reflected light intensity decrease, which can make dew point determination difficult.